Accurate and rapid collection and distribution of geophysical property data is a key to successful exploration and production of petroleum resources. Based on data such as electrical and nuclear properties collected in a well-bore, as well as the propagation of sound through a formation, geophysicists make an analysis useful in making many important operational decisions. The analysis includes determination of whether a well is likely to produce hydrocarbons, whether to drill additional wells in the vicinity of an existing well, and whether to abandon a well as being unproductive. Geophysicists may also use well-bore data to select where to set casing in a well and to decide on how to perforate a well to stimulate hydrocarbon flow. One method of collecting well-bore geophysical properties is by way of well-logging. In well-logging, a well-logging tool (also often referred to as a sonde) is lowered into a well-bore on an electrical cable, the wireline. The well-logging tool is an electrically powered measurement device that may, for example, collect electrical data, sonic waveforms that are propagated through the surrounding formation, or radioactivity counts. These measurements are usually converted to a digital form and transmitted on the wireline. Systems for transmitting data from the well-logging tool to a data acquisition system over a wireline cable are known as wireline telemetry systems.
One prior art wireline telemetry system is the Digital Telemetry System of Schlumberger Technology Corporation. U.S. Pat. No. 5,838,727 describes DTS. DTS, like other prior art wireline telemetry systems, transmits data from the logging tool to the surface computer using a single carrier frequency. Using this single carrier system, the maximum throughput is approximately 500,000 bits per second. Modern logging tools are capable of generating data at much higher rates. Therefore, the prior art telemetry systems present a bottleneck in the delivery of well-logging data.
Wireline cables are primarily designed for mechanical properties. A modern oil well may be drilled to a depth of in excess of 30,000 feet. The cable must be able to sustain the tension generated from the weight of the logging tools and the weight of the lengthy cable itself. Electrical properties, on the other hand, are given a lower priority. Therefore, wireline cables are not ideal conveyors of the information that is transmitted from the well-logging tools. It is desirable to provide wireline telemetry systems that can be tailored for specific or individual cables and conditions to maximally use the data delivery capabilities of a specific wireline cable.
Using a formula, known as Shannon's capacity formula, it is possible to determine a theoretical maximum channel capacity of a communication channel given a certain level of noise. Prior art well-logging telemetry systems achieve data rates that are considerably lower than the theoretical capacity. While it may not be practical (or even possible) to build a system that does achieve the Shannon capacity, it is nevertheless desirable to provide a system that achieves a data rate that comes as close as possible to the Shannon capacity for a given wireline cable.
Because of the electrical limitations on a wireline cable, the signal-to-noise ratio can be unacceptably high and significantly impact the data rate. It would be desirable to provide a system and method which overcomes the signal-to-noise ratio problems associated with wireline telemetry systems.
From the foregoing it will be apparent that there is still a need for a way to obtain a higher throughput in a wireline telemetry system and to have a telemetry system that is more adaptable to the specific characteristics of the wireline cable.
The problem of obtaining high data throughput in a communications system also exists in Internet communication. One recently popular technology for high-speed Internet is known as ADSL (Asymmetric Digital Subscriber Line). One modulation technique used in ADSL is Discrete Multi-Tone modulation (DMT). An advantage of ADSL is the high throughput. With ADSL it is possible to obtain a data rate of over 4.5 million bits per second on a twisted-pair telephone line.
Hitherto DMT has not been used in wireline telemetry systems because of inherent difficulties in applying DMT to wireline. These difficulties arise in part from high signal attenuation on wireline cables. It would be advantageous to overcome the problems of applying DMT to wireline telemetry and thereby obtain the benefits of ADSL and DMT in well-lagging data acquisition.